Many chemical reactions are difficult to carry out in the lab under normal conditions. The reasons for this are
varied but might include:
This means that we cannot measure the enthalpy changes for these reactions directly. Luckily
we can use Hess's law to
calculate the enthalpy changes for these reactions.
- the reaction might be very slow and take a long time to complete.
- there maybe unwanted side reactions taking place
Hess's Law is simply an extension of the law of conservation of energy,
that is that energy cannot be created or destroyed only
change from one form to another. As an example consider the image opposite. Here the change A→B is an exothermic
reaction with an enthalpy change (ΔH1) of -1000kJ. Now imagine that for some reason the change A→B cannot be
carried out directly. However the change A→C and C→B is possible with enthalpy changes
of ΔH2= -400kJ and
ΔH3= -600kJ. This route from A→B simply goes through an intermediate C. However the important
fact is that both reactions, the direct route
A→B and the other route through intermediate C both start and end up at the same place.
They both start at A and end up at B.
In essence this is Hess's law. Hess's law
The enthalpy change for a reaction depends only on the
initial and final states of the reaction and is independent
of the route by which the reaction occurs.
So in our example:
Or we could write ΔH1 = ΔH2 + ΔH3.
- the direct route A→B has an enthalpy change of ΔH1= -1000kJ.
- the indirect route A→C has an enthalpy change of ΔH2= -400kJ and C→B
has an enthalpy change of ΔH3= -600kJ.
That is the sum of the two enthalpy changes, ΔH2 + ΔH3 is the same as the
route ΔH1. Which is just another way of saying that the enthalpy change for a reaction is equal to the sum of the
enthalpy changes for any individual steps, as long as you start and end points
are the same. That's all there is to Hess's law!
Worked examples using Hess' law
Examiners often ask you to calculate the enthalpy change for a reaction using either enthalpies of combustion (ΔcH
or enthalpies of formationn(ΔfH o). If you are
not sure what these enthalpy changes represent then click the links to do a little research before continuing.
Using enthalpies of combustion (ΔcH
o) to calculate an enthalpy change
||Enthalpy of combustion/kJmol-1
Example 1 - Calculate the enthalpy of
formation of methane (CH4(g)) using the enthalpies of combustion given in the table opposite.
All the questions you are likely to be asked to solve are all done the same way. So follow the method below and you can't
Step 1 - You must write the equation for the reaction you are being asked to calculate the
enthalpy change for. In this case we
have to calculate the enthalpy of formation of methane. An equation for
this enthalpy change is given below:
C(s) + 2H2(g) → CH4(g)
To calculate this enthalpy change we need the enthalpies of combustion
of all the element and compound present
in this equation, a quick
check in the table shows that you have all these enthalpies of combustion.
Now all we need to do is draw a simple energy cycle using the data we have.
Some students get confused on how to start these cycles, but if you are using enthalpies of combustion,
which we are here, then the cycle
Using enthalpies of formation (ΔfH
calculate an enthalpy change
The other type of problem you are likely to meet is where you have to use enthalpies of formation
to calculate an enthalpy change.
As before the first step is to write out the equation you have been asked to find the
enthalpy change for.
||Enthalpy of formation/kJmol-1
Example 2- Calculate the enthalpy change for the following reaction using the
enthalpy of formation(ΔfH
o) data in the table.
4NH3(g) + 5O2(g) → 4NOg) + 6H2O(l)
As before in order to calculate the enthalpy of reaction (ΔrxnH) we need to
know the enthalpy of formation (
(ΔfH) for all the substances in our equation. Looking at the table we have all the
information we need, remember
the enthalpy of formation of an element is by definition 0.
When we drew the energy cycle above using enthalpies of combustion (ΔcH
o) we drew the cycle
"downwards" from our equation. This time using enthalpies of formation (ΔfH o) we
will work "upwards" towards our equation.
The hardest part of getting started with a Hess's law energy cycle is
knowing where to start - remember that
if you are given enthalpies of combustion data to calculate an unknown
enthalpy change your energy cycle goes "downwards".
If you are given enthalpies of formation data to calculate an unknown
enthalpy change your energy cycle